Biology Reference
In-Depth Information
Fig. 5.5
terms of the effect produced by a mutation in that gene; this name is written by con-
vention in
italics
. Figure 5.5 compares the head of an
eyeless
mutant of
Drosophila
with that of the normal, wild type fly. It was seventy-six years later before a similar
mutation was found in mice, and this gene was called
Small eye.
Mice that contain
one copy of the mutant gene have reduced eyes, but those with two copies lack eyes
entirely, as well as the nose and a large part of the forebrain, and die before birth.
The right-hand part of Figure 5.5 shows the effect of this mutation in mice embryos.
This mutation in mice resembles a rare human genetic condition called aniridia,
where one copy of the mutant gene causes reduced irises, but two copies result in no
eyes or nose, and early death. The key connection between these genes was made
in the 1990s when it was discovered that the amino acid sequence encoded by the
eyeless
,
Small eye
and
aniridia
genes are very similar to one another and to a similar
gene in zebrafish, amphibians, sea urchins, squid and planarians.
This was a surprising finding because until that time the very different eye struc-
tures found in vertebrates and invertebrates had supported the idea that they had
independent origins. This discovery prompted the different view that
Pax6
is a mas-
ter regulatory gene involved in determining eye development in most, if not all,
animals, and thus that there was some common ancestry in the early evolution of
different types of eye. This change of view is another example of the provisional
nature of all scientific ideas. You will recall that I pointed out in Chapter 2 that all
